A typical solid polymer electrolyte membrane fuel cell with a single cell assembly includes a housing with (i) a membrane electrode assembly which comprises a solid polymer electrolyte membrane sandwiched between an anode and a cathode, (ii) an anode fluid flow plate, and (iii) a cathode fluid flow plate. The anode fluid flow plate contains an anode flow field with reactant channels for distributing a fuel to the anode. The cathode fluid flow plate has a cathode flow field with reactant channels for distributing an oxidant to the cathode. The fluid flow plates also separate the fuel from the oxidant during cell operation and provide an electrical connection between the anode and the cathode. During operation, appropriate fuel and oxidant are supplied to the anode flow field and the cathode flow field, respectively, the fuel and the oxidant react at the electrodes to generate electrical current, and product water is produced and withdrawn from the cell.
A typical solid polymer electrolyte membrane solid fuel cell having a "stack assembly" operates in a similar manner in a housing with at least two membrane electrode assemblies that are operatively associated with a plurality of "bi-polar" plates, i.e. plates having an anode flow field on one major face of the plate and a cathode flow field on the opposite side.
Conventional solid polymer electrolyte membrane (SPE) fuel cells utilizing hydrogen and air as reactants have several limitations that have made them unsuitable in many applications. Conventional hydrogen/air fuel cells are typically cooled by liquid cooling systems that increase both the manufacturing and the operating costs of the fuel cells. U.S. Pat. No. 5,230,966, for example, discloses a liquid coolant field plate with a coolant network means and a means for attaching the plate to a cell. Japanese Patent 92-175748 discloses a fuel cell having a cooling plate sandwiched between a pair of separator plates. It would be desirable to develop a fuel cell that is air-cooled and that operates without a liquid cooling system.
"Water management" is an important problem in fuel cell design. Water management involves providing the cell with a sufficient amount of water to humidify the membrane and anode. Water management also involves removing water from the cell to prevent the product water from agglomerating and clogging reactant channels. To humidify the membrane and an anode during operation, conventional SPE cells need to draw water from external sources. In a conventional hydrogen/air fuel cell, the membrane would dehydrate if it is operated at a relatively high current. As the membrane dries, the internal resistance of the cell increases, and the power output of the cell is substantially reduced. It is also well known that an anode's resistance to the transport of ions increases if the anode dries. Water must be introduced into the cell to avoid this from happening. U.S. Pat. No. 4,824,741, for instance, discloses a SPE fuel cell having a means for pumping water into a porous anode field plate to moisten and cool the electrolyte membrane. Water is also introduced into the cell by combining hydrogen (or air) with water. It would be desirable to develop a fuel cell that does not require systems to introduce water into the cell.
To avoid clogged reactant channels, compressors are frequently used to exhaust product water generated in conventional SPE cells. In a typical SPE cell, product water (if not removed) clogs cathode reactant channels and lowers the performance of the cell. To avoid this from happening, compressors exhaust product water from the cell. The use of a compressor adds expense and complexity to the system. It would be desirable to develop a fuel cell that does not require the use of compressors to exhaust product water from the cell.
Accordingly, it is object of this invention to develop a fuel cell that overcomes the above problems.
It is a further object of this invention to develop a fuel cell that is air cooled and that operates without a liquid cooling system.
It is a further object of this invention to develop a fuel cell that does not require water from an external source to be introduced into the cell.
It is a further object of this invention to develop a fuel cell that does not require compressors to exhaust product water from channels during fuel cell operation.
It is a further object of this invention to develop a fuel cell that is self-humidifying.
It is a further object of this invention to develop a method for using the above-named fuel cells.
These and still further objects are apparent from the following description of this invention.